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Scientists Discover Receptor for Carbonation Taste

Almost 250 years after chemist Joseph Priestley infused water
with carbon dioxide to make the first artificially carbonated
water, researchers have finally discovered how people “taste” that
carbonation bubbling in their beverage.

Over the past decade, researchers have made tremendous progress
identifying the basis for detecting the 5 taste qualities—sweet,
sour, salty, bitter and savory. Scientists believe that our perception
of flavors arise from this limited palate of tastes, along with
input from other senses like touch and smell. Interestingly,
the gas carbon dioxide also induces strong responses in taste
nerves, but how it's detected by the taste system was unclear.

The laboratories of Dr. Nicholas Ryba of NIH's National Institute
of Dental and Craniofacial Research (NIDCR) and Dr. Charles Zuker
from the Howard Hughes Medical Institute at the University of
California at San Diego previously teamed up to identify the
components responsible for our sweet, bitter, savory and sour
detection. In their new study, appearing in the October 16, 2009,
edition of Science, they explored the taste of carbonation
by recording the electrical responses of taste nerves in mice.

The team tested mice engineered to lack different types of
taste cells and discovered that sour-sensing cells were responsible
for detecting carbon dioxide. Further work revealed that the
response is initiated by an enzyme called carbonic anhydrase
4 (CA-IV). CA-IV is one of a family of enzymes that helps convert
carbon dioxide to carbonic acid, providing cells and tissues
with a buffer to help prevent excessive changes in pH. CA-IV
interacts with carbon dioxide on the surface of sour-sensing
cells in taste buds, the researchers found, prompting sour cells
to send a message to the brain.

“Of course, this raises the question of why carbonation
doesn't just taste sour,” Ryba says. He says the answer
likely lies in the somatosensory system, which transmits information
about touch, pain and temperature to the brain. “We know
that carbon dioxide also stimulates the mouth's somatosensory
system. Therefore, what we perceive as carbonation must reflect
the combination of this somatosensory information with that from
taste.”

The body senses carbon dioxide on many levels—in the
somatosensory system, smell, and in the brain and blood to control
respiration. But why would mammals need to taste carbon dioxide?
One possibility is that it might serve an important role—for
instance, as a way to avoid fermenting foods, which have high
levels of carbon dioxide.

“Although we don't know for certain,” Ryba says, “we
favor the explanation that carbon dioxide detection by taste
may be accidental, occurring simply because of the presence of
CA-IV on the surface of the sour cells. We think CA-IV is there
to maintain the pH balance and health of the taste buds rather
than to act as a carbon dioxide detector.”

If this explanation turns out to be true, then the familiar,
unique and attractive sensation we get when we sip a carbonated
drink is just a fortunate consequence of a much more fundamental
biological need.